The present invention relates to a process for making hermetically sealed filter units; in particular, to a process for molding a porous filter element to a thermoplastic support to insure an integral seal. The present invention also relates generally to hermetically sealed filter units and, more specifically, to such filters which are used to remove bacteria and other minute contaminants from a fluid stream and which include a novel arrangement of structural features to improve both their function and utility.
Molded plastic filters and filter units are well-known and enjoy a wide use and general acceptance in a variety of industries. The function of such units is to remove contaminants from liquid or gaseous materials which flow there through. One important application of such filter units is to remove bacteria, gases and other contaminants from solutions being introduced into the human bloodstream. In critical applications of this nature, it is imperative that the filter media not be damaged and that the filtrate not by-pass the filtering medium, for either situation would allow contaminants to enter the bloodstream. It is necessary, therefore, that an undamaged, hermetic seal exist to prevent leakage around the filter element and also to prevent possible bacterial contamination from outside the assembly.
Another important function of such filter units is to purge the air or other gases which may be entrapped in the solution. Thus, filters used to purify intravenous solutions must provide some mechanism for venting such entrapped air from the upstream side of the filter media while maintaining the hermetic seal of the media. Similarly, the filter unit must be constructed to assure the complete removal of any entrapped gases at the entry side of the filter element prior to introduction of the filtrate into the human bloodstream.
Typically, the filter units used to filter intravenous solutions comprise a two-part housing and an internally disposed filter sandwiched there between.
Several methods of sealing the filter elements have been employed in the past. These methods include pressure clamping, heat sealing, ultrasonic welding, and adhesive and solvent bonding. These prior art methods, however, may fail to provide, on occassion, the one hundred percent positive seal which is necessary to prevent the leakages described above. Moreover, in critical applications where special microporous membranes are used, there is some danger when using these prior art methods that delicate filter media or thin plastic sealing flanges will be damaged during the sealing process.
Pressure clamping and other mechanical interlocking systems may distort the filter media or actually damage the filter media at the clamping edges, thereby destroying the integrity of the filter media and allowing contaminants to pass through. Also, many conditions such as time, and heat stress relieving can allow the pressure seal to relax and allow contaminants to by-pass the filter media.
Heat sealing, sonic welding and related thermo-mechanical bonding methods may also damage the filter medium and are suspect to reliability especially when the filter unit has an uneven, irregular or unsymetrical shape.
The use of adhesives or solvent bonding has disadvantages in that another material is introduced that can lead to contamination on its own. Often the constituents of an adhesive or solvent system may be damaging to the microporous membrane.
Thus, there is a need for a new process which will provide a completely hermetic seal for plastic molded filter units and which will insure that the filter medium is completely sealed to the filter housing without any damage to the structural integrity of even the most delicate components.
In addition, since the filter media utilized in such filter units may be quite delicate, it is desirable to provide support to this element on the downstream or low pressure side. Such supportive structures used in prior art devices, while generally acceptable, tend to obstruct the fluid flow passageway on the downstream side of the filter media which obstruction, in turn, results in the entrapment of small gas bubbles which are sometimes difficult to purge. There is a need, therefore, for a filter unit which avoids these structural shortcomings found in prior art devices and which thereby assures the expeditious and complete purging of downstream gases from the filter unit prior to filtrate use.